1. Field
Embodiments of the present invention relate to laser systems and, in particular, to tunable lasers systems.
2. Discussion of Related Art
An optical telecommunication system transmits information from one place to another by way of an optical carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, light beam, or a lightwave signal. The optical telecommunication system includes several optical fibers and each optical fiber includes multiple channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. The purpose for using multiple channels in the same optical fiber (called dense wavelength division multiplexing (DWDM)) is to take advantage of the unprecedented capacity (i.e., bandwidth) offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap. The International Telecommunications Union (ITU) currently determines the channel separations.
One link of an optical telecommunication system typically has a transmitter, the optical fiber, and a receiver. The transmitter has a laser, which converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
External cavity lasers (ECL) and distributed feedback (DFB) lasers are common light sources. While such lasers have typically operated at a single wavelength or channel or a small number of channels, widely tunable lasers have been recently developed that can address many channels, for example, at least all channels in one of the communication frequency bands specified by the ITU.
One difficulty in exploiting laser tunability is in guaranteeing wavelength accuracy. One aspect of wavelength accuracy is the degree to which the lasing wavelength corresponds to one of the pre-defined channels. Aging of the laser or changes in the environment can cause the laser to drift in frequency, resulting in sub-optimal performance in the fiber optic network.
Components known as wavelength lockers have been employed to combat wavelength drift. Known wavelength lockers fall into two categories: periodic and monotonic.
Known periodic wavelength lockers provide error signals to correct laser output frequency to one of the evenly spaced ITU optical frequency channels. Periodic wavelength lockers do not distinguish between the channels, however. This means that known periodic wavelength lockers provide signals to lock onto a channel but the channel may be an incorrect channel.
Known monotonic wavelength lockers also provide error signals to correct laser output frequency to one of the evenly spaced ITU optical frequency channels. The response of monotonic wavelength lockers varies so strongly with wavelength that ITU channels can be uniquely identified. To be able to uniquely identify a channel, monotonic wavelength lockers can only resolve a few channels and cannot span a full ITU band of channels.
Thus, a servo system based on known wavelength lockers will typically cause the laser wavelength to be updated to lock to the nearest pre-defined channel, which may be the wrong channel. This error will not be detected and will result incorrect routing of any data on the light beam.